The A subunit of vacuolar H+-ATPase gene (CmVHA-A) plays opposite roles in plant growth and drought tolerance of chrysanthemum under different growing conditions.

As the most prominent proton pumps in plants, vacuolar H+-ATPases (VHAs) comprise multiple subunits that are important for physiological processes and stress tolerance in plants. However, few studies on the roles of subunit genes of VHAs in chrysanthemum have been reported to date. In this study, the gene of A subunit of V-ATPase in chrysanthemum (CmVHA-A) was cloned and identified. CmVHA-A was conserved with VHA-A proteins from other plants. Expression analysis showed that CmVHA-A was highly expressed in most tissues of chrysanthemum except for the flower bud, and was readily induced by polyethylene glycol (PEG) treatment. Functional analysis demonstrated that CmVHA-A exerted a negative influence on the growth and development of shoot and root of chrysanthemum under normal conditions. RNA-sequencing (RNA-seq) analysis revealed the possible explanations for phenotypic differences between transgenic and wild-type (WT) plants. Under drought conditions, CmVHA-A positively affected the drought tolerance of chrysanthemum by enhancing antioxidase activity and alleviating photosynthetic disruption. Overall, CmVHA-A plays opposite roles in plant growth and drought tolerance of chrysanthemums under different growing conditions.

The BTB/TAZ domain-containing protein CmBT1-mediated CmANR1 ubiquitination negatively regulates root development in chrysanthemum.

Roots are fundamental for plants to adapt to variable environmental conditions. The development of a robust root system is orchestrated by numerous genetic determinants and, among them, the MADS-box gene ANR1 has garnered substantial attention. Prior research has demonstrated that, in chrysanthemum, CmANR1 positively regulates root system development. Nevertheless, the upstream regulators involved in the CmANR1-mediated regulation of root development remain unidentified. In this study, we successfully identified bric-a-brac, tramtrack and broad (BTB) and transcription adapter putative zinc finger (TAZ) domain protein CmBT1 as the interacting partner of CmANR1 through a yeast-two-hybrid (Y2H) screening library. Furthermore, we validated this physical interaction through bimolecular fluorescence complementation and pull-down assays. Functional assays revealed that CmBT1 exerted a negative influence on root development in chrysanthemum. In both in vitro and in vivo assays, it was evident that CmBT1 mediated the ubiquitination of CmANR1 through the ubiquitin/26S proteasome pathway. This ubiquitination subsequently led to the degradation of the CmANR1 protein and a reduction in the transcription of CmANR1-targeted gene CmPIN2, which was crucial for root development in chrysanthemum. Genetic analysis suggested that CmBT1 modulated root development, at least in part, by regulating the level of CmANR1 protein. Collectively, these findings shed new light on the regulatory role of CmBT1 in degrading CmANR1 through ubiquitination, thereby repressing the expression of its targeted gene and inhibiting root development in chrysanthemum.

MdbHLH3 modulates apple soluble sugar content by activating phosphofructokinase gene expression.

Sugars are involved in plant growth, fruit quality, and signaling perception. Therefore, understanding the mechanisms involved in soluble sugar accumulation is essential to understand fruit development. Here, we report that MdPFPß, a pyrophosphate-dependent phosphofructokinase gene, regulates soluble sugar accumulation by enhancing the photosynthetic performance and sugar-metabolizing enzyme activities in apple (Malus domestica Borkh.). Biochemical analysis revealed that a basic helix-loop-helix (bHLH) transcription factor, MdbHLH3, binds to the MdPFPß promoter and activates its expression, thus promoting soluble sugar accumulation in apple fruit. In addition, MdPFPß overexpression in tomato influenced photosynthesis and carbon metabolism in the plant. Furthermore, we determined that MdbHLH3 increases photosynthetic rates and soluble sugar accumulation in apple by activating MdPFPß expression. Our results thus shed light on the mechanism of soluble sugar accumulation in apple leaves and fruit: MdbHLH3 regulates soluble sugar accumulation by activating MdPFPß gene expression and coordinating carbohydrate allocation.

Glucose sensor MdHXK1 activates an immune response to the fungal pathogen Botryosphaeria dothidea in apple.

Sugars are essential regulatory molecules involved in plant growth and development and defense response. Although the relationship between sugars and disease resistance has been widely discussed, the underlying molecular mechanisms remain unexplored. Ring rot caused by Botryosphaeria dothidea (B. dothidea), which severely affects fruit quality and yield, is a destructive disease of apples (Malus domestica Borkh.). The present study found that the degree of disease resistance in apple fruit was closely related to glucose content. Therefore, the gene encoding a hexokinase, MdHXK1, was isolated from the apple cultivar 'Gala', and characterized during the defense response. Overexpression of MdHXK1 enhanced disease resistance in apple calli, leaves and fruits by increasing the expression levels of genes related to salicylate (SA) synthesis (PHYTOALEXIN DEFICIENT 4, PAD4; PHENYLALANINE AMMONIA-LYASE, PAL; and ENHANCED DISEASE SUSCEPTIBILITY 1, EDS1) and signaling (PR1; PR5; and NONEXPRESSER OF PR GENES 1, NPR1) as well as increasing the superoxide (O2- ) production rate and the hydrogen peroxide (H2 O2 ) content. Overall, the study provides new insights into the MdHXK1-mediated molecular mechanisms by which glucose signaling regulates apple ring rot resistance.

Yang cycle enzyme DEP1: its moonlighting functions in PSI and ROS production during leaf senescence.

Ethylene-mediated leaf senescence and the compromise of photosynthesis are closely associated but the underlying molecular mechanism is a mystery. Here we reported that apple DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1 (MdDEP1), initially characterized to its enzymatic function in the recycling of the ethylene precursor SAM, plays a role in the regulation of photosystem I (PSI) activity, activating reactive oxygen species (ROS) homeostasis, and negatively regulating the leaf senescence. A series of Y2H, Pull-down, CO-IP and Cell-free degradation biochemical assays showed that MdDEP1 directly interacts with and dephosphorylates the nucleus-encoded thylakoid protein MdY3IP1, leading to the destabilization of MdY3IP1, reduction of the PSI activity, and the overproduction of ROS in plant cells. These findings elucidate a novel mechanism that the two pathways intersect at MdDEP1 due to its moonlighting role in destabilizing MdY3IP1, and synchronize ethylene-mediated leaf senescence and the compromise of photosynthesis.

Mechanisms and regulation of organic acid accumulation in plant vacuoles.

In fleshy fruits, organic acids are the main source of fruit acidity and play an important role in regulating osmotic pressure, pH homeostasis, stress resistance, and fruit quality. The transport of organic acids from the cytosol to the vacuole and their storage are complex processes. A large number of transporters carry organic acids from the cytosol to the vacuole with the assistance of various proton pumps and enzymes. However, much remains to be explored regarding the vacuolar transport mechanism of organic acids as well as the substances involved and their association. In this review, recent advances in the vacuolar transport mechanism of organic acids in plants are summarized from the perspectives of transporters, channels, proton pumps, and upstream regulators to better understand the complex regulatory networks involved in fruit acid formation.

New insights into the role of MADS-box transcription factor gene CmANR1 on root and shoot development in chrysanthemum (Chrysanthemum morifolium).

BACKGROUND: MADS-box transcription factors (TFs) are the key regulators of multiple developmental processes in plants; among them, a chrysanthemum MADS-box TF CmANR1 has been isolated and described as functioning in root development in response to high nitrate concentration signals. However, how CmANR1 affects root and shoot development remains unclear. RESULTS: We report that CmANR1 plays a positive role in root system development in chrysanthemum throughout the developmental stages of in vitro tissue cultures. Metabolomics combined with transcriptomics assays show that CmANR1 promotes robust root system development by facilitating nitrate assimilation, and influencing the metabolic pathways of amino acid, glycolysis, and the tricarboxylic acid cycle (TCA) cycle. Also, we found that the expression levels of TFs associated with the nitrate signaling pathways, such as AGL8, AGL21, and LBD29, are significantly up-regulated in CmANR1-transgenic plants relative to the wild-type (WT) control; by contrast, the expression levels of RHD3-LIKE, LBD37, and GATA23 were significantly down-regulated. These results suggest that these nitrate signaling associated TFs are involved in CmANR1-modulated control of root development. In addition, CmANR1 also acts as a positive regulator to control shoot growth and development. CONCLUSIONS: These findings provide potential mechanisms of MADS-box TF CmANR1 modulation of root and shoot development, which occurs by regulating a series of nitrate signaling associated TFs, and influencing the metabolic pathways of amino acid and glycolysis, as well as TCA cycle and nitrate assimilation.

R2R3-MYB Transcription Factor MdMYB73 Confers Increased Resistance to the Fungal Pathogen Botryosphaeria dothidea in Apples via the Salicylic Acid Pathway.

MYB transcription factors (TFs) participate in many biological processes. However, the molecular mechanisms by which MYB TFs affect plant resistance to apple ring rot remain poorly understood. Here, the R2R3-MYB gene MdMYB73 was cloned from "Royal Gala" apples and functionally characterized as a positive regulator of the defense response to Botryosphaeria dothidea. qRT-PCR and GUS staining demonstrated that MdMYB73 was strongly induced in apple fruits and transgenic calli after inoculation with B. dothidea. MdMYB73 overexpression improved resistance to B. dothidea in apple calli and fruits, while MdMYB73 suppression weakened. Increased resistance to B. dothidea was also observed in MdMYB73-expressing Arabidopsis thaliana. Interestingly, salicylic acid (SA) contents and the expression levels of genes related with SA synthesis and signaling were greater in MdMYB73-overexpressing plant materials compared to wild-type controls after inoculation, suggesting that MdMYB73 might enhance resistance to B. dothidea via the SA pathway. Finally, we discovered that MdMYB73 interacts with MdWRKY31, a positive regulator of B. dothidea. Together, MdWRKY31 and MdMYB73 enhanced B. dothidea resistance in apples. Our results clarify the mechanisms by which MdMYB73 improves resistance to B. dothidea and suggest that resistance may be affected by regulating the SA pathway.

The apple bHLH transcription factor MdbHLH3 functions in determining the fruit carbohydrates and malate.

Changes in carbohydrates and organic acids largely determine the palatability of edible tissues of horticulture crops. Elucidating the potential molecular mechanisms involved in the change in carbohydrates and organic acids, and their temporal and spatial crosstalk are key steps in understanding fruit developmental processes. Here, we used apple (Malus domestica Borkh.) as research materials and found that MdbHLH3, a basic helix-loop-helix transcription factor (bHLH TF), modulates the accumulation of malate and carbohydrates. Biochemical analyses demonstrated that MdbHLH3 directly binds to the promoter of MdcyMDH that encodes an apple cytosolic NAD-dependent malate dehydrogenase, activating its transcriptional expression, thereby promoting malate accumulation in apple fruits. Additionally, MdbHLH3 overexpression increased the photosynthetic capacity and carbohydrate levels in apple leaves and also enhanced the carbohydrate accumulation in fruits by adjusting carbohydrate allocation from sources to sinks. Overall, our findings provide new insights into the mechanism of how the bHLH TF MdbHLH3 modulates the fruit quality. It directly regulates the expression of cytosolic malate dehydrogenase MdcyMDH to coordinate carbohydrate allocation and malate accumulation in apple.

The basic helix-loop-helix transcription factor MdbHLH3 modulates leaf senescence in apple via the regulation of dehydratase-enolase-phosphatase complex 1.

Basic helix-loop-helix (bHLH) domain-containing transcription factors are known for their roles in regulating various plant growth and developmental processes. Previously, we showed that MdbHLH3 from apple (Malus domestica) has multiple functions, modulating both anthocyanin biosynthesis and cell acidification. Here, we show that MdbHLH3 also regulates ethylene biosynthesis and leaf senescence by promoting the expression of dehydratase-enolase-phosphatase complex 1 (MdDEP1). Therefore, we propose a model whereby MdbHLH3 acts as a crucial factor that modulates anthocyanin biosynthesis and cell acidification in addition to fruit ripening and leaf senescence by regulating distinct target genes.

CmTCP20 Plays a Key Role in Nitrate and Auxin Signaling-Regulated Lateral Root Development in Chrysanthemum.

Lateral root (LR) formation and development play a vital role in plant development by permitting the establishment of branched root systems. It is well known that nutrient availability controls LR development. Moreover, LR development is fine-tuned by a myriad of hormonal signals. Many transcription factors (TFs) participate in LR development. Here, we discuss the TFs involved in the nitrate and auxin signaling pathways and how these function in the regulation of LR formation and development in chrysanthemum. AtTCP20 is a plant-specific TF, which can modulate LR development in response to nitrate. The roles of CmTCP20 in LR development were identified by overexpression in chrysanthemum and heterologous expression in Arabidopsis. Overexpression of CmTCP20 significantly increased the number and average length of LRs compared with the wild type in chrysanthemum and Arabidopsis. We also found that CmTCP20 positively influenced auxin accumulation in the LRs at least partly by improving auxin biosynthesis, transport and response, thereby promoting LR development. Moreover, we found that CmTCP20 interacts with an auxin response factor, CmARF8, which also can be induced by nitrate and combined to proximal sites in the upstream promoter region of CmCYCB1;1 to positively regulate the cell cycle. The CmTCP20-CmARF8 heterodimer links nitrate and auxin signaling and converts cell-cycle signals to regulate LR initiation and growth.

The regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple.

The plant hormone ethylene is critical for climacteric fruit ripening, while glucose and anthocyanins determine the fruit quality of climacteric fruits such as apple. Understanding the exact molecular mechanism for this process is important for elucidating the interconnection of ethylene and fruit quality. Overexpression of apple MdbHLH3 gene, an anthocyanin-related basic helix-loop-helix transcription factor (bHLH TF) gene, promotes ethylene production, and transgenic apple plantlets and trees exhibit ethylene-related root developmental abnormalities, premature leaf senescence, and fruit ripening. Biochemical analyses demonstrate that MdbHLH3 binds to the promoters of three genes that are involved in ethylene biosynthesis, including MdACO1, MdACS1, and MdACS5A, activating their transcriptional expression, thereby promoting ethylene biosynthesis. High glucose-inhibited U-box-type E3 ubiquitin ligase MdPUB29, the ortholog of Arabidopsis AtPUB29 in apple, influences the expression of ethylene biosynthetic genes and ethylene production by direct ubiquitination of the MdbHLH3 protein. Our findings provide new insights into the ubiquitination of MdbHLH3 by glucose-inhibited ubiquitin E3 ligase MdPUB29 in the regulation of ethylene biosynthesis as well as indicate that the regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple.

The MADS transcription factor CmANR1 positively modulates root system development by directly regulating CmPIN2 in chrysanthemum.

Plant root systems are essential for many physiological processes, including water and nutrient absorption. MADS-box transcription factor (TF) genes have been characterized as the important regulators of root development in plants; however, the underlying mechanism is largely unknown, including chrysanthemum. Here, it was found that the overexpression of CmANR1, a chrysanthemum MADS-box TF gene, promoted both adventitious root (AR) and lateral root (LR) development in chrysanthemum. Whole transcriptome sequencing analysis revealed a series of differentially expressed unigenes (DEGs) in the roots of CmANR1-transgenic chrysanthemum plants compared to wild-type plants. Functional annotation of these DEGs by alignment with Gene Ontology (GO) terms and biochemical pathway Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that CmANR1 TF exhibited "DNA binding" and "catalytic" activity, as well as participated in "phytohormone signal transduction". Both chromatin immunoprecipitation-polymerase chain reaction (ChIP-PCR) and gel electrophoresis mobility shift assays (EMSA) indicated the direct binding of CmPIN2 to the recognition site CArG-box motif by CmANR1. Finally, a firefly luciferase imaging assay demonstrated the transcriptional activation of CmPIN2 by CmANR1 in vivo. Overall, our results provide novel insights into the mechanisms of MADS-box TF CmANR1 modulation of both AR and LR development, which occurs by directly regulating auxin transport gene CmPIN2 in chrysanthemum.

Ectopic expression of the apple nucleus-encoded thylakoid protein MdY3IP1 triggers early-flowering and enhanced salt-tolerance in Arabidopsis thaliana.

BACKGROUND: The roles in photosystem I (PSI) assembly of the nucleus-encoded thylakoid protein Y3IP1 who interacts with the plastid-encoded Ycf3 protein that has been well-characterized in plants. However, its function and potential mechanisms in other aspects remain poorly understood. RESULTS: We identified the apple MdY3IP1 gene, which encodes a protein highly homologous to the Arabidopsis Y3IP1 (AtY3IP1). Ectopic expression of MdY3IP1 triggered early-flowering and enhanced salt tolerance in Arabidopsis plants. MdY3IP1 controlled floral transition by accelerating sugar metabolism process in plant cells, thereby influencing the expression of flowering-associated genes. The increase in salt stress tolerance in MdY3IP1-expressing plants correlated with reduced reactive oxygen species (ROS) accumulation, and an increase in lateral root development by regulating both auxin biosynthesis and transport, as followed by enhancement of salt tolerance in Arabidopsis. Overall, these findings provide new evidences for additional functions of Y3IP1-like proteins and their underlying mechanisms of which Y3IP1 confers early-flowering and salt tolerance phenotypes in plants. CONCLUSIONS: These observations suggest that plant growth and stress resistance can be affected by the regulation of the MdY3IP1 gene. Further molecular and genetic approaches will accelerate our knowledge of MdY3IP1 functions in PSI complex formation and plants stress resistance, and inform strategies for creating transgenic crop varieties with early maturity and high-resistant to adverse environmental conditions.

Chrysanthemum MADS-box transcription factor CmANR1 modulates lateral root development via homo-/heterodimerization to influence auxin accumulation in Arabidopsis.

Root system architecture is an important agronomic trait by which plants both acquire water and nutrients from the soil and adapt to survive in a complex environment. The adaptation of plant root systems to environmental constraints largely depends on the growth and development of lateral roots (LRs). MADS-box transcription factors (TFs) are important known regulators of plant growth, development, and response to environmental stimuli. However, the potential mechanisms by which they regulate LRs development remain poorly understood. Here, we identified a MADS-box chrysanthemum gene CmANR1, homologous to the Arabidopsis gene AtANR1, which plays a key role in the regulation of LR development. qRT-PCR assays indicated that CmANR1 was primarily expressed in chrysanthemum roots and was rapidly induced by exposure to high nitrate concentrations. Ectopic expression of CmANR1 in Arabidopsis significantly increased the number and length of emerged LRs compared to the wild-type (col) control, but had no obvious affect on primary root (PR) development. We also found that CmANR1 positively influenced auxin accumulation in LRs at least partly by improving auxin biosynthesis and transport, thereby promoting LR development. Furthermore, we found that ANR1 formed homo- and heterodimers through interactions with itself and AGL21 at its C-terminal domain. Overall, our findings provide considerable new information about the mechanisms by which the chrysanthemum MADS-box TF CmANR1 mediates LR development by directly altering auxin accumulation.

Nitrate: A Crucial Signal during Lateral Roots Development.

Root plasticity is an important trait for plants to forage nutrient and adapt to survival in a complicated environment. Lateral roots (LRs) are generally more sensitive than primary roots in response to changing environmental conditions. As the main source of nitrogen for most higher plants, nitrate acting as a signal has received great attention in the regulation of LR development. In general, there are dual effects including stimulatory and inhibitory of low nitrate on LR development; while high nitrate supply has an inhibitory effect on LR development; nitrate heterogeneity also has a stimulatory effect on LR development in [Formula: see text]- rich zone. Here, we focus on recent progresses in the role of a nitrate signal in the regulation of the LRs development.

The R2R3-MYB transcription factor MdMYB73 is involved in malate accumulation and vacuolar acidification in apple.

Malate, the predominant organic acid in many fruits, is a crucial component of the organoleptic quality of fruit, including taste and flavor. The genetic and environmental mechanisms affecting malate metabolism in fruit cells have been studied extensively. However, the transcriptional regulation of malate-metabolizing enzymes and vacuolar transporters remains poorly understood. Our previous studies demonstrated that MdMYB1 modulates anthocyanin accumulation and vacuolar acidification by directly activating vacuolar transporters, including MdVHA-B1, MdVHA-E, MdVHP1 and MdtDT. Interestingly, we isolated and identified a MYB transcription factor, MdMYB73, a distant relative of MdMYB1 in this study. It was subsequently found that MdMYB73 protein bound directly to the promoters of MdALMT9 (aluminum-activated malate transporter 9), MdVHA-A (vacuolar ATPase subunit A) and MdVHP1 (vacuolar pyrophosphatase 1), transcriptionally activating their expression and thereby enhancing their activities. Analyses of transgenic apple calli demonstrated that MdMYB73 influenced malate accumulation and vacuolar pH. Furthermore, MdCIbHLH1 interacted with MdMYB73 and enhanced its activity upon downstream target genes. These findings help to elucidate how MdMYB73 directly modulates the vacuolar transport system to affect malate accumulation and vacuolar pH in apple.

Glucose Sensor MdHXK1 Phosphorylates and Stabilizes MdbHLH3 to Promote Anthocyanin Biosynthesis in Apple.

Glucose induces anthocyanin accumulation in many plant species; however, the molecular mechanism involved in this process remains largely unknown. Here, we found that apple hexokinase MdHXK1, a glucose sensor, was involved in sensing exogenous glucose and regulating anthocyanin biosynthesis. In vitro and in vivo assays suggested that MdHXK1 interacted directly with and phosphorylated an anthocyanin-associated bHLH transcription factor (TF) MdbHLH3 at its Ser361 site in response to glucose. Furthermore, both the hexokinase_2 domain and signal peptide are crucial for the MdHXK1-mediated phosphorylation of MdbHLH3. Moreover, phosphorylation modification stabilized MdbHLH3 protein and enhanced its transcription of the anthocyanin biosynthesis genes, thereby increasing anthocyanin biosynthesis. Finally, a series of transgenic analyses in apple calli and fruits demonstrated that MdHXK1 controlled glucose-induced anthocyanin accumulation at least partially, if not completely, via regulating MdbHLH3. Overall, our findings provide new insights into the mechanism of the glucose sensor HXK1 modulation of anthocyanin accumulation, which occur by directly regulating the anthocyanin-related bHLH TFs in response to a glucose signal in plants.

MdSOS2L1 forms a complex with MdMYB1 to control vacuolar pH by transcriptionally regulating MdVHA-B1 in apples.

Vacuolar pH is important and involves in many different physiological processes in plants. A recent paper published in Plant Physiology reveals that MdMYB1 regulates vacuolar pH by directly transcriptionally regulating proton pump genes and malate transporters genes, such as V-ATPase subunit gene MdVHA-B1. Here, we found that MdSOS2L1 in vitro did not directly interact with MdMYB1, however, in vivo formed a complex with MdMYB1 in the nucleus to regulate MdVHA-B1-mediated vacuolar acidification. This finding shed light on the role of MdSOS2L1 in transcriptionally regulating MdVHA-B1 in addition to its post-modified function in apples.